Film Structure and Adhesion

Paint films although attached to a substrate on one side only may be subjected to stresses, comparable to those in structural adhesives. These stresses result from shrinkage during film formation and subsequent ageing, mechanical strains, relative thermal movements of film and substrate and from osmotic pressure due to soluble material under or within the film. The adhesive strength required to prevent detachment varies from very little for weak, highly porous coatings to 10,000 lb/in² for tough coatings of high elastic modulus. Generally, adhesive strength both to the substrate and between coats in a paint system must exceed cohesive strength, under the conditions when failure is likely to develop. Dispersion and other forces, such as hydrogen bridging, between coatings and clean metal substrates should suffice to ensure adhesion but most practical surfaces carry contaminants, which interfere with wetting and intimacy of contact. Solvents and other low molecular weight components may also provide a weak interfacial layer, at least for a period after application. Modification of polymer structure to improve contaminant displacement and to increase polymer/substrate interaction forces, for example by the introduction of polar substituent or end groups will be discussed and potentialities of adhesion-promoting surface treatments reviewed.     

Molecular simulations of deformation and failure in bonds formed by glassy polymer adhesives

The mechanical behavior of glassy polymer bonds is examined with molecular dynamics simulations. We show that the interfacial strength of the bond in mode I (tensile) and mode II (shear) fracture is strongly influenced by the coupling between the adhesive and adherends as well as by the roughness of the substrate surface. Failure occurs at the substrate (interfacial failure) when the interaction is weak, and in the bulk (cohesive failure) when the interaction is strong. The transition from interfacial to cohesive failure under mode I loading is nearly unaffected by roughness, while roughness leads to a dramatic increase in interfacial strength under mode II loading. Stress mixity is another crucial parameter that determines whether the polymer fails through shear deformation or through cavitation and crazing. By varying the geometry of the adhesive bond, we illustrate different limiting behaviors of a rupturing film.     

Low Dielectric Polyimide/Fluorinated Ethylene Propylene (PI/FEP) Nanocomposite Film for High-Frequency Flexible Circuit Board Application

The low dielectric polymer films have drawn great attention to the application as the dielectric insulating materials in high-frequency circuit boards, while the weak adhesion to the copper foils and the poor processability resulted from the fluorinated or rigid structures limited their high-frequency application. In this work, the low dielectric and high adhesive polyimide/fluorinated ethylene propylene (PI/FEP) nanocomposite film for high-frequency flexible circuit board application is developed. It is indicated that the fluorocarbon surfactants can significantly improve the dispersion of FEP in PI substrate, and thus, the PI/FEP nanocomposite film exhibits excellent mechanical properties, including the tensile strength increases to 46.6 MPa and the elongation at the break increases to 13.7%. Importantly, at the high-frequency of 10 GHz, the 60 wt% FEP filled PI nanocomposite film displays an ultralow dielectric loss (0.006) and a reduced dielectric constant (2.69). In addition, the high-frequency flexible circuit board with the PI/FEP film as the dielectric insulating layer has a high peel strength of 0.75 N mm⁻¹, indicating this PI/FEP nanocomposite film can meet the requirements of the high-frequency flexible circuit board application.     

Effect of certain hostile environments on adhesive joints

The effects of water vapour on aluminium-aluminium bonded joints, using a Bisphenol A diglycidyl ether epoxide resin cured with bis-(4-aminophenyl)-methane as the adhesive, have been studied. A distinction has been drawn between the effect on joint strength and on polymer strength. The very significant fall in joint strength compared with the change in polymer strength is used to postulate that in adhesive joints of this type, water molecules rapidly enter the boundary region to interfere with the secondary valence forces responsible for adhesion between substrate and organic adhesive. The results are further interpreted as support for the view that the polymer adjacent to the metal substrate is likely to differ in structure and conformation from the main polymer mass, namely a boundary layer. Similar experiments using the less polar ethanol provide further evidence for the hypotheses.     

Film formation and crack development in plasma polymerized hexamethyldisiloxane

Plasma polymer thin coatings can be used in a variety of applications that require thin ultrasmooth, defect-free homogeneous films. In this study both film formation and crack development are described and related to the structure of the plasma polymer. The growth of a plasma polymer film reproduces the topography of the porous (polysulfone) PSf support and the time needed for complete coverage depends on the rate of deposition which is a function of the plasma power and monomer flow rate. The plasma polymerized hexamethyldisiloxane, PPHMDSO, film on a PSf porous support is under compressive internal stress causing the membrane to bend convexly with respect to the film. Despite these stresses, the composite membrane (film/support) did not crack when freely immersed in water and alcohols. Cracks developed in the mechanically constrained PPHMDSO composite membrane on exposure to alcohols. Films with more organic character showed high adhesion among the plasma polymer nodules and between the plasma polymer and the PSf substrate, resulting in substrate tearing and cracking straight through the plasma polymer layer.     

Effect of Deformation-induced Residual Stress on Peel Strength of Polymer Laminated Sheet Metal

The adhesion between adhesively bonded polymer film and a metallic sheet substrate in a polymer laminated sheet metal (PLSM) subjected to large deformation, such as in a forming process, is influenced by two deformation-induced factors. These are (i) evolution of surface roughness of metallic substrate with applied strain and (ii) development of residual stress in the polymer adherend (polymer film with a thin uniform adhesive layer on one side) arising from significant differences in the deformation behavior of metal and polymeric components. A new experimental methodology was devised in this study to decouple the effects of substrate surface roughness and residual stress on interfacial peel strength (IPS) of uniaxially deformed PLSMs. This methodology was based on 180° peel testing of PLSM specimens prepared under two different lamination conditions, one involving systematic pre-straining in uniaxial tension of the metallic substrate prior to laminations and the other involving post-lamination pre-straining of the PLSM. The role of pre-strain and peel test speed, for the above laminations conditions, were critically analyzed for their effect on IPS of two differently tailored PLSM systems. The IPS results were attributed to the effect of deformation-induced residual stress and metallic surface roughness. The analysis suggests that IPS is strongly dependent upon the residual stress induced by uniaxial deformation but only marginally on substrate surface roughness depending upon the constituents (film and adhesive) of the adherend. The magnitude of pre-strain was inversely and non-linearly related to IPS for both deformed PLSMs. Peel test speed, on the other hand, showed a more complex behavior in terms of IPS for the two PLSM systems.     

Specific Interactions, Initial and Equilibrium Bond Strength in Polymer/Polymer Assemblies

This paper reports on bonding characteristics of assemblies using as substrates poly(vinyl chloride) (PVC), acrylonitrile-butadiene-styrene (ABS) and polypropylene (PP), and as melt adhesives an ethylene-vinyl acetate (EVA) copolymer, a polyurethane (PUr), and low density polyethylene (LDPE). Peel strength measurements on freshly assembled joints were compared with results for samples aged under inert and humid conditions. Significant time-dependent variations of bond strength were observed in all cases, but the direction of change varied among the assemblies. Those involving only dispersion-force materials displayed losses of bond strength, attributable to the gradual accumulation of cohesively weak layers at the substrate/adhesive interface. In assemblies involving materials able to interact by non-dispersion (acid/base) forces, as indicated by inverse gas chromatographic data, a variety of responses was obtained. These have been rationalized by the ability of the EVA and PUr adhesives to reorient when in contact with an appropriate polymer substrate. Reorientation, leading to bond strength increments, was associated with substrate/adhesive pairs (e.g., PVC/EVA and ABS/PUr), in which significant acid/base interaction could take place.     

AlCrN/Cr3C2–NiCr duplex coating towards high load-bearing and dry sliding antiwear applications

In this study, experimental analysis and finite element modeling (FEM) were employed to investigate the microstructure and mechanical properties of duplex coatings composed of a Cr₃C₂–NiCr interlayer and a top AlCrN film in comparison with these of a single AlCrN film. Results showed a significant improvement in the adhesive strength, load-bearing capacity, H/E, and H³/E² ratios, and hardness of the AlCrN/Cr₃C₂–NiCr duplex coatings compared to the single AlCrN film, especially the wear resistance that increased by nearly eight times under heavy loads. Moreover, FEM analysis revealed that the duplex design reduced the stress concentration area on the surface of AlCrN film and kept it far away from the contact interface during load-bearing.     

Adhesion of polymer coils to a solid surface: influence of the depletion effect

The specific properties of polymer coils are often disregarded in theories of adhesion, but polymer properties are essential for the strength of the adhesive bond. Polymer coils are repelled entropically from impenetrable surfaces. This causes the depletion effect and creates a layer of reduced concentration right at the interface. To bond a polymer coil to a substrate, it must be forced actively towards the interface, driven by the gaining of adsorption energy. The adsorption of specific groups in the (co)polymer, which interact with 'polar' sites on the substrate, must be used to suppress the depletion. Adsorption diminishes the effective distance between the surface and the adhesive polymer. The balance between adsorption and depletion (rather than the effect of polar groups or pretreatments on the work of adhesion as such) is the most important chemical possibility of affecting adhesion. The strength of the bond between polymeric materials and solid surfaces varies as H^-3, with the effective distance H between the polymer and substrate. Therefore, it changes by an order of magnitude when the polymer adhesive is pulled towards the substrate by adsorption.     

A Method of Predicting the Stresses in Adhesive Joints after Cyclic Moisture Conditioning

The durability of adhesive joints is of special concern in structural applications and moisture has been identified as one of the major factors affecting joint durability. This is especially important in applications where joints are exposed to varying environmental conditions throughout their life. This paper presents a methodology to predict the stresses in adhesive joints under cyclic moisture conditioning. The single lap joints were manufactured from aluminium alloy 2024 T3 and the FM73®-BR127® adhesive-primer system. Experimental determination of the mechanical properties of the adhesive was carried out to measure the effect of moisture uptake on the strength of the adhesive. The experimental results revealed that the tensile strength of the adhesive decreased with increasing moisture content. The failure strength of the single lap joints also progressively degraded with time when conditioned at 50°C, immersed in water; however, most of the joint strength recovered after drying the joints. A novel finite element based methodology, which incorporated moisture history effects, was adopted to determine the stresses in the single lap joints after curing, conditioning, and tensile testing. A significant amount of thermal residual stress was present in the adhesive layer after curing the joints; however, hygroscopic expansion after the absorption of moisture provided some relief from the curing stresses. The finite element model used moisture history dependent mechanical properties to predict the stresses after application of tensile load on the joints. The maximum stresses were observed in the fillet areas in both the conditioned and the dried joints. Study of the stresses revealed that degradation in the strength of the adhesive was the major contributor in the strength loss of the adhesive joints and adhesive strength recovery also resulted in recovered joint strength. The presented methodology is generic in nature and may be used for various joint configurations as well as for other polymers and polymer matrix composites.     

Adhesion between polymer substrates and plasma films from tetramethylsilane and tetramethyltin by glow discharge polymerization

Adhesion between various polymer substrates and plasma films, which had been prepared from either tetramethylsilane or tetramethyltin by glow discharge polymerization and deposited on the surface of the polymer, was evaluated by the Scotch tape test and by lap-shear strength. It was found that the plasma films exhibited fairly good adhesion to the polymer substrates (with the exception of polypropylene). The position where failure occurred was determined by X-ray fluorescence analysis, scanning electron microscopy and energy diffractive X-ray analysis. This position was at an inner layer of the plasma film (cohesive failure of plasma film), within the polymer substrate (material failure of polymer) or at the interface between polymer substrate and plasma film (adhesive failure) depending upon the polymer substrate. These results indicate an important aspect of durability of surface modification by glow discharge polymerization.     

Polymer structure and properties of heterophase and self-stratifying coatings

Phase separations in bulk and during the film forming process on a substrate, accompanied with the evaporation of solvents and chemical reactions of curing, have been studied in incompatible polymer blends (combinations of solid epoxy resin with selected thermoplastic resins, priorly dissolved in organic solvents) in order to be able to control the heterophase polymer structure of coatings. The resultant polymer/ polymer heterogeneous and sometimes non-homogeneous-in-layer (ultimately double-layer) polymer structures of coatings were evaluated by scanning electron microscopy (SEM). A few driving forces for self-stratification and the formation of double-layer coating structures, where each layer is enriched with a certain polymer component, are discussed as possibly associated with selective wetting of a substrate, phase contraction, interfacial tension gradients, etc. Examples are given to illustrate non-additive changes of structure-dependent coating properties with the composition of the epoxy/thermoplastic resin blends, which is characteristic for polymer/polymer heterophase materials. Depending on the phase state of the binder in the paint (formulated with the use of a selected incompatible polymer blend, combination of solvents and sometimes with an additive, controlling the phase separation process) and conditions of application to a substrate and film formation, various polymer/polymer heterophase or self-stratifying one-coat coatings can be obtained. Some of them, particularly those that combine partial stratification with polymer/polymer heterogeneity, are capable of meeting the demands for advanced performance due to improved combinations of bulk and surface properties.     

AFM Scratching for Adhesion Studies of Thin Polymer Coatings on Steel

Abstract

Atomic force miscroscopy (AFM) scratching at constant applied forces was used to quantify the adhesion of polymer coatings to cold rolled steel (CRS) and to study the effectiveness of a pretreatment for improving the adhesion. The pretreatment was a phosphate-free zirconia-based coating. Thin layers of commercially available epoxy, acrylic and polyester-based polymer coatings, were applied to polished or pretreated cold rolled steel substrates and the surface was scratched at the edge of the polymer coatings with the AFM tip at increasing values of normal loads until the coating was removed. Adhesion strengths were determined from the minimum tip-sample interaction force and number of cycles (scans) at a particular applied force. The pretreatment significantly improved adhesion of the epoxy and acrylic-based coatings on CRS. Adhesion of the acrylic-based coating was found to be better than the epoxy coatings on the bare as well as pretreated steel. Adhesive strength of the polyester-based coating was inconclusive because it was very easily removed on application of small forces using the AFM tip. The AFM scratching technique was found to provide a quick, easy and effective way to make quantifiable comparisons in relative adhesive strengths of polymer coatings and the effect of pretreatments.     

Zinc deposited polymer coatings for copper protection

Polypyrrole (PPy) and polyaniline (PAni) coatings were electrosynthesized on copper, by using cyclic voltammetry technique. Then, these coatings were modified with the deposition of zinc particles from aqueous zinc sulphate solution. The electrodeposition of zinc was achieved at a constant potential value of −1.20 V, in the amount of ∼0.75 mg/cm². The corrosion performance of zinc modified polymer coatings were investigated in 3.5% NaCl solution; by using the electrochemical impedance spectroscopy (EIS), and anodic polarization curves. The zinc particles improved the barrier property of polymer films, thanks to formation of voluminous zinc corrosion products within the pores of polymer coating. Also, the zinc particles provided cathodic protection to the substrate, where the polymer film played the role of conductance between zinc particles and copper.     

Guidelines for dryer design based on results from non‐Fickian model

In polymer solution coatings below the glass transition temperature of the pure polymer, the coating can go undergo a glass transition and develop stresses during drying. When stresses develop, a non‐Fickian model accurately describes solvent mass transport in drying polymer coatings. The non‐Fickian model includes the solvent transport due to both stress and concentration gradients. This article presents a non‐Fickian model, which predicts a lower residual solvent than does the corresponding Fickian model. We showed in an earlier article that the non‐Fickian model predicts trapping skinning (higher residual solvent under more intense operating conditions) at higher drying gas‐flow rates. In this article, the non‐Fickian model was used to investigate how the gas‐flow rate, dry film thickness, and substrate thickness affect the residual solvent for a single‐zone dryer. This work recommends guidelines for choosing gas‐flow rates, gas temperatures, and substrate thickness to minimize the residual solvent. The model predictions show that, at any gas temperature, the residual solvent is minimum at an intermediate gas‐flow rate. The trapping skinning effect is less evident in thicker coatings and substrates. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 477–486, 2003     

Peel‐strength behavior of bilayer thermal‐sprayed polymer coatings

We prepared various bilayer polymer coatings of ethylene methacrylic acid (EMAA) copolymer and ionomer by the thermal‐spray process under a range of preheat temperatures (PTs) to investigate their ability to be repaired. The thermal properties, crystallinity, microstructure, and interface strength of the coatings were investigated with differential scanning calorimetry, X‐ray diffraction, scanning electron microscopy, and mechanical testing. Processing parameters influenced the final morphological structure of the coatings. The crystallinity of the coatings increased with a higher final temperature, whereas the coating density decreased. The decrease in density was attributed to the appearance of bubbles, 250 μm in size, formed in the coatings during the spray process. For the monolayer coating of polymer on a metal substrate, a higher PT produced a greater contact area of the coating to the substrate. The adhesion of EMAA ionomer to steel was always lower than that of EMAA copolymer to steel. This may have been largely due to the interfacial adhesion between the polymer and steel being dominated by strong secondary bond interactions. Experimental results also indicate that the peel strength between polymers was at least twofold stronger than that between the polymer and the steel substrate for PTs greater than 100°C. The mixed bilayer coating of ionomer on copolymer produced the highest peel strength. The interface between the plastic layers was clearly visible under the scanning electron microscope at lower PTs, becoming more diffuse with an increase in PT. On the basis of these observations, the adhesion mechanism between polymers was explained by the formation of welding points. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 214–226, 2003     

Biocoatings: challenges to expanding the functionality of waterborne latex coatings by incorporating concentrated living microorganisms

Biocoatings concentrate living, nongrowing microbes in nanoporous adhesive polymer films. Any microbial activity or trait of interest can be intensified and stabilized in biocoatings. These films will dramatically expand the functionality of waterborne coatings. Many microbes contain enzyme systems which are unstable when purified. Therefore, thin polymer coatings of active microbes are a revolutionary approach to stabilize living cells as industrial or environmental biocatalysts. We have demonstrated that some microbes survive polymer film formation embedded in nontoxic adhesive waterborne binders by controlling formulation and drying. Biocoatings can be a single layer of randomly oriented microbes or highly structured multilayer films combining monolayers of different types of microbes on solid, porous, or flexible substrates. They can be formed by drawdown or ink-jet deposition, convective sedimentation assembly, dielectrophoresis, or coated onto or embedded within papers. Controlled drying generates nanoporous microstructure; the pores are filled with a carbohydrate glass which stabilizes the entrapped dehydrated microbes. When the coating is rehydrated, the carbohydrates diffuse out generating nanopores. The activity of biocoatings can be 100s of g L^?1 (coating volume) h^?1 stabilized for 100–1000s of hours, and therefore, they represent a new approach to process intensification (PI) using thin liquid film bioreactors. A current challenge is that many microbes being engineered as environmental, solar, or carbon recycling biocatalysts do not naturally survive film formation. The mechanisms of dehydration damage that occur during biocoating formulation, ambient drying, and during dry storage have begun to be studied. Critical to preserving microbe viability are minimizing osmotic stress, toxic monomers, biocides, and utilizing polymer chemistries that generate strong wet adhesion with arrested coalescence (nanoporosity). Therefore, controlling desiccation, drying rate/uniformity, and residual moisture are important. Optimization of biocoating activity can be affected at multiple stages—cellular engineering prior to coating (preadaptation), formulation, deposition (film thickness), film formation/drying (generates microstructure), dry storage (minimize metabolic activity), and rehydration. Gene induction (activation) leading to enzyme synthesis following rehydration has been demonstrated. However, little is known about gene regulation in nongrowing microbes. Challenges to optimizing biocoating activity include generating stable film porosity, strong wet adhesion, control of residual water content/form/distribution, and nondestructive measurement of entrapped microbe viability and activity. Indirect methods to measure viability include vital staining, enzyme activity, reporter genes, response to light, confocal fluorescent microscopy, and ATP content. Microbes containing stress-inducible reporter genes can be used to monitor cell stress during formulation, film formation, and drying. Future cellular engineering to optimize biocoatings includes desiccation tolerance, light reactivity (photoefficiency), response to oxidative stress, and cell surface-to-polymer or substrate adhesion. Preservation of microbial activity in waterborne coatings could lead to high intensity biocatalysts for environmental cleaning, gaseous carbon recycling, to produce H₂ or electricity from microbial fuel cells, delivery of probiotics, or for biosolar energy harvesting.     

Addition of precipitated calcium carbonate filler to thermoplastic polyurethane adhesives

Precipitated calcium carbonate filler (PCC) was added to a thermoplastic polyurethane adhesive (TPU). The addition of PCC produced a moderate increase in the rheological and viscoelastic properties of TPU due to the poor dispersion of filler (i.e. found to be clusters) and the weak interactions between the PCC nanoparticles and the polymer chains. The interactions were noticed by ATR-IR spectroscopy by displacement of the bands at 3326, 1729 and 1061 cm⁻¹ to higher wave number of the polyurethane. Furthermore, the first glass transition temperature of the polyurethane was found to decrease by adding PCC filler. The crystallinity of the soft segments in the TPU was decreased by adding PCC because of the disruption of the degree of phase separation in the polymer. The initial adhesive strength in PVC/TPU adhesive/PVC joints increased noticeably by adding PCC filler, the greater the amount of filler in the TPU, the greater the initial adhesive strength found. Finally, the highest final adhesive strength (72 h after joint formation) was obtained in the joint produced with the TPU containing 10 wt% PCC.     

β-TCP coatings on zirconia bioceramics: The importance of heating temperature on the bond strength and the substrate/coating interface

β-tricalcium phosphate (β-TCP) coatings were synthesized on tetragonal zirconia (Y-TZP) discs by heating the apatite coating between 800?°C and 1200?°C. The study results suggest that heating temperature has a strong influence on the coating bond strength and microstructure of the substrate/coating interface. The β-TCP coatings fired at 800?°C and 900?°C exhibited excellent tensile bond strength (～50?MPa) while heating at 1100?°C and 1200?°C led to decreased bond strength (～30?MPa) as the result of substantial structural and microstructural changes: diffusion of Y³⁺ from the zirconia substrate in the coating resulting in partial crystal transformation (t-m) of zirconia, formation of surface uplifts and nanoporosity in zirconia, as well as generation of large residual thermal stresses leading to microcracking of the β-TCP coatings. However, these structural changes did not have any measurable effect on the flexural strength of the bulk zirconia substrates.     

Tailoring adhesive forces between poly(dimethylsiloxane) and glass substrates using poly(vinyl alcohol) primers

A thin poly(vinyl alcohol) (PVA) layer has been found to control adhesive forces between poly(dimethylsiloxane) (PDMS) and a glass substrate. Various PVAs were coated on glass substrates on top of which PDMS pre‐polymer was cast. After thermal curing, the peel strength was tested. It was found that the fundamental adhesive forces are attributed to the degree of hydrolysis (or saponification value) of the PVAs. For a PVA modified with a silanol group, strong adhesive force resulted. The range of tailoring the force with the PVAs was 16 kgf/m. The production of thin interlaminated PVA layers as primers was demonstrated. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39927.